Device for determining local absorption values in a slice of a body, and an array of detectors for such a device

ABSTRACT

An X-ray examining device for transversal tomography, comprising an array of detectors of mutually different dimensions. Each detector comprises one or more detector elements which operate electrically in parallel, either as a result of the direct parallel connection of the detector elements or by the addition of output signals of integrators which are connected to each detector element. It is possible to connect more or fewer detector elements in parallel by means of switches, so that it is possible to select the resolution of the array of detectors.

This is a continuation of 898,790, filed on Aug. 18, 1986, which is acontinuation of application Ser. No. 123,638, filed Feb. 22, 1980, whichis a continuation of Ser. No. 948,282, filed Oct. 3, 1978, allabandoned.

BACKGROUND OF THE INVENTION

Device for determining local absorption values in a slice of a body, andan array of detectors for such a device.

The invention relates to a device for determining local radiationabsorption values in a slice of a body, comprising at least oneradiation source for generating a fan-shaped beam of radiation whichirradiates the body, and an array of detectors for detecting radiationwhich passes through the body in various directions, the direction ofthe radiation being situated in the slice, the radiation source and thearray of detectors being situated opposite each other on each side of acentral axis through the body to be examined, the detectors on and neara connecting line between radiation source, central axis and array ofdetectors having a smaller detection surface area than the detectorswhich are situated further from the connecting line, a detector signalfrom a detector being electrically isolated from the detector signalfrom any other detector. The invention furthermore relates to an arrayof detectors for such a device.

A device of the described kind is known from U.S. Pat. No. 3,973,128.This patent describes a detector construction which has a spatialresolution, in the absorption distribution of the irradiated slice to bereconstructed, which is position-dependent. The detector constructioncomprises detectors whose detection surface areas facing the body arenot equal. At the ends of the array of detectors, the detection surfaceareas are larger than the detection surface areas in the center of thearray, with the result that the spatial resolution in the center of thereconstructed absorption distribution is higher than that at the edge.

SUMMARY OF THE INVENTION

An object of the invention is to provide a device in which the area ofthe detection surfaces of the detectors (i.e. the spatial resolution inthe ultimate reconstruction of the absorption distribution) can beadapted to the nature of the body to be examined.

To this end, the device according to the invention is characterized inthat the array of detectors comprises, viewed in the direction of thearray, a series of consecutively arranged detector elements which haveuniform dimensions and behavior. The detection surface area of adetector, made up of one or more detector elements, is thereforeproportional to an integral multiple of the detection surface area of adetector element. The detector signal is proportional to the sum of theoutput signals of the detector elements comprising the detector.

Because all detector elements are uniform, the manufacture of an arrayof detectors consisting of such detector elements will be cheaper thanthe manufacture of an array of detectors having unequal detectionsurface areas.

The parallel operation of the detector elements need not be permanent.As a result, groups of detector elements can operate in parallelaccording to the nature of the body to be examined and the requiredquality of the image of the slice of the body to be examined. It hasbeen found that this is an attractive feature, because for examinationsperformed on the human torso the required resolution may usually belower than that required, for example, for basicranial examinations.

The use of an array of detectors having different dimensions createsproblems, notably when ionization chambers are used as detectors.Ionization chambers of different dimensions have different responsetimes and non-linearity. This makes calibration and processing of thedetector signals difficult.

To this end, a preferred embodiment of a device according to theinvention is characterized in that the detector elements are ionizationchambers. The construction of detectors having unequal detection surfaceareas from one or more ionization chambers, each of which has a uniformbehavior, results in each detector having the same response time andnon-linearity, which is advantageous for the necessary calibration ofthe array of detectors.

In a further embodiment of the device according to the invention, theoutputs of the ionization chambers associated with a detector areinterconnected via electrically conductive connection means. The signalsupplied by the detector is the sum of the ionization currents of theindividual, parallelly connected ionization chambers. A knownpre-amplifier which acts as an integrator may be connected to theparallel connection of the ionization chambers.

Another preferred embodiment of the device according to the invention ischaracterized in that each detector element is connected to anintegrator, there being provided per detector an adding circuit wheretooutputs of the integrators of the ionization chambers associated withthe detector are connected via electrical connection means. In thisembodiment, the connection means are preferably constructed as switches.Such a set-up of the electrical circuit connected to the detectorelements offers the advantage that the susceptibility to interference isless in comparison with a construction in which switches are directlyconnected to the ionization chambers. Because the ionization currentshave already been integrated before they are applied to an addingcircuit via switches, the circuit is less susceptible to the electricalirregularities formed by the switches therein. An array of detectors,which is composed of a series of uniform ionization chambers and inwhich the spatial resolution is changed by the switching of switches, isattractive notably where many different kinds of examinations areperformed. For example, the device according to the invention can beused to examine part of the slice to be examined with a high spatialresolution, while the surrounding parts are examined with a lowerspatial resolution. Furthermore, the same device can be used to examinethe entire slice with a low resolution (for example, during X-rayexamination of large organs, such as lungs or liver) or with a highresolution (for example, basicranial X-ray examinations).

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in detail hereinafter with reference tothe accompanying diagrammatic drawing.

FIG. 1 shows a device according to the invention;

FIG. 2 shows a preferred embodiment of a part of a schematic diagram ofa series of ionization chambers for a device as shown in FIG. 1, and

FIG. 3 shows a further embodiment of a schematic diagram of a series ofdetector elements for a device as shown in FIG. 1.

FIG. 4 is a schematic diagram of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 diagrammatically shows an X-ray examining device according to theinvention, comprising a radiation source 1 which preferably consists ofan X-ray tube, but which may alternatively consist of, for example, aradioactive isotope such as Am241 or Gd153. An array of detectors 3 (forexample, comprising 300 detectors) locally measures the intensity of thebeam of radiation 2 to be emitted by the radiation source. Radiation 2is preferably X-radiation. The radiation source in this case forms afan-shaped beam having an angle of aperture α which amounts to, forexample, 60°. The beam is substantially in planes parallel to the planeof the drawing and has a small thickness of, for example, from 3 to 15mm perpendicular to the plane of the drawing. A slit-like aperture 4 isprovided for the formation of such a beam. The width dimension of thedetectors 3 and the distance therebetween determine the feasible spatialresolution within a given beam angle of the fan-shaped beam 2. Asupporting table 7, on which a body 8 to be examined is arranged, islongitudinally displaceable along a central axis 9 which is directedperpendicularly to the plane of the drawing. The system formed by theX-ray source 1 and the array of detectors 3 can be rotated around thebody 8 by means of a toothed ring 10 which is driven by a motor 11 andwhich is supported by guides 12. Rotation of the system formed by theX-ray source 1 and the array of detectors 3 may be continuous as well asintermittent. In the latter case a rotation step is formed after eachmeasurement. A counter 18 counts the number of detector signals receivedper measurement by an arithmetic device 15. When a number is reachedwhich corresponds to the number of detectors, the control circuit 19 ofthe motor 11 is actuated for a brief period of time, so that a rotationstep takes place. The system formed by the X-ray source 1 and the arrayof detectors 3 is suspended in a frame 20. The frame 20 is movable alongguide rollers 21 by means of a motor 22, so that the X-ray source 1 canbe moved away from or towards the body along a central connecting line6. It is thus achieved that the beam 2 generated by the X-ray source 1,having an apex α, can always exactly cover the body 8, so that optimumuse is made of the array of detectors 3 during the examination. Prior tothe start of a measurement, the distance between the X-ray source 1 andthe body 8 is adjusted, for example, by switching on the control circuit24 by hand.

Each of the detectors 5, 5a and 5b is connected, via a cable bundle 13,to an amplifier/converter 14 in which the detector signals areindividually processed. The amplifier/converter 14 may comprise, forexample, a multiplex circuit and an analog-to-digital converter. Theoutput of the amplifier/converter 14 is connected to an arithmeticdevice 15, whereby the local absorption is calculated on the basis ofthe amplified and converted detector signals. The calculated absorptionvalues are stored in a memory device 16 and, if desired, displayed on adisplay device 17.

The array of detectors 3 according to the invention consists of an arrayof adjacently arranged detector elements which have uniform dimensionsand behavior. Preferably, the detector elements are ionization chambersfilled with a rare gas, such as xenon, and an extinction gas. Theionization chambers are accommodated, for example, in a gastight housingand are formed by plate-shaped, parallelly arranged, electricallyconductive electrodes as have been proposed in U.S. patent applicationSer. No. 895,706, filed Apr. 12, 1978 and assigned to the assignee ofthe instant application. The detectors 5, situated around the centralconnecting line 6 in the center of the X-ray beam 2, comprise, forexample, one ionization chamber. Some detectors 5a which are situated oneither side of detectors 5 each comprise two parallelly connectedionization chambers. Each of the detectors 5b situated at the ends ofthe array comprises four parallelly connected ionization chambers. Ifthe array of detectors 3 comprises, for example, 384 ionization chambersand the angle of aperture is 48°, a practical construction of thedetectors is as follows: On either side of the central connecting line 6from

0°-15°: 8 detectors per degree (1 ionization chamber per detector)

15°-18°: 4 detectors per degree (2 ionization chambers per detector)

18°-21°: 2 detectors per degree (4 ionization chambers per detector)

21°-24°: 1 detector per degree (8 ionization chambers per detector)

The total number of detectors then amounts to 282, while the totalnumber of ionization chambers amounts to 384.

The signal-carrying electrodes of the ionization chambers (i.e. theoutput signals of the detector elements) associated with a detector canbe readily interconnected electrically. A further possibility ofrealizing parallel operation of detector elements is shown in FIG. 2.The detector elements to be used have been proposed in U.S. patentapplication Ser. No. 885,670, filed Mar. 13, 1978 and assigned to theassignee of the instant application. A series of detector elements 30a .. . e, each of which comprises a semiconductor diode, is connected to apower supply source 29. The detector elements 30a . . . e shown formonly a fraction of the number of detector elements used. Each of thedetector elements 30a . . . d, together constituting a detector, isconnected to an integrator 31a . . . d which, by way of illustration,comprises an operational amplifier and a capacitor. The outputs of theintegrators 31a . . . d are connected to an adding circuit 33. Theadding circuit 33 comprises, for example, input resistors 35a . . . dwhich all have the same resistance, an operational amplifier 37, and afeedback resistor 39. The output signal of the adding circuit 33 is thedetector signal of a detector which comprises four detector elementsoperating in parallel. Depending on the position of a detector in thearray of detectors 3, 1, 2, 4 or 8 detector elements are connected to anadding circuit.

FIG. 3 shows a schematic diagram for a series of ionization chambers41a, b; 43a, b; 45a . . . h; 47a, b and 49a, b for a device according tothe invention. FIG. 3 only shows a number of ionization chambers (16)which is small in comparison with the often more than 300 ionizationchambers used in practice. The a-sections and the b-sections of theionization chambers 41a, b; 43a, b; 47a, b and 49a, b are permanentlyconnected in parallel and form four detectors. The ionization chambersare symmetrically arranged with respect to the central connecting line 6which is also shown in FIG. 1. By means of a two-position switch 40, theionization chambers can be connected to the integrators 51a . . . h intwo ways. In the position of the switch 40 shown, the ionizationchambers 45a and b form a detector which is connected to the integrator51c. Each of the ionization chambers 45c and d, e and f, g and h alsoforms a detector which is connected to the integrators 51d, e and f,respectively. The array of detectors thus comprises 8 detectors, each ofwhich comprises two parallelly connected ionization chambers.

When the switch 40 is switched over, the ionization chambers 45a, b, c,d, e, f, g and h are connected to the integrators 51a, b, c, d, e, f, gand h, respectively. The array of detectors in this configurationcomprises 8 detectors, each of which comprises one ionization chamber.Therefore, switching over can be used to make a choice between a longarray of detectors (low resolution) and a short array of detectors (highresolution), the number of detectors being the same in both arrays. Theshort array of detectors will enclose a smaller angle α (see FIG. 1)than a long array of detectors. The apex α of the radiation beam 2 canbe adapted to the length of the array of detectors 3 by using theappropriate apertures 4.

In FIG. 3, the switch 40 is connected directly to the detector elements(ionization chambers). Obviously, it is alternatively possible toconnect an integrator 51 to each detector element (as in FIG. 2) and toconnect a switch 40 to the outputs thereof in order to establish andinterrupt the desired connections between the outputs of the integratorsand the adding circuits. (FIG. 4.) Obviously, the resistor of the addingcircuit may also be permanently connected to each output of theintegrators, a switch such as the switch 40 being connected to saidresistors in order to establish the desired connections to the remainderof the adding circuits.

What is claimed is:
 1. A device for determining local radiationabsorption values in a slice of a body having a central axis,comprising:at least one radiation source for generating a fan-shapedbeam of radiation which irradiates the slice of the body and whichpasses through the slice in different directions, said radiation sourcebeing situated on a first side of the central axis; an array of detectorelements, having uniform dimensions and behavior, for detecting theradiation emitted by the radiation source, said array being situated ona second side of the central axis opposite said radiation source, eachof said detector elements generating an output signal; means connectedto the output signal from each detector element for separatelytime-integrating the output signals from each detector element; meansfor adding the integrated output signals from a group of detectorelements, the sum being an integrated detector signal which iselectrically isolated from the integrated detector signal of any otherdetector element or group of detector elements; and switchable means forconnecting the integrated output signals from at least two groups ofdetector elements to the adding means, said switchable means connectingthe integrated output signals of only one group of detector elements ata time.
 2. An array of detector elements, comprising:a series ofionization chambers having uniform dimensions and behavior, each of saidionization chambers generating an output signal; means for adding theoutput signals from a group of one or more ionization chambers, the sumbeing a detector signal which is electrically isolated from the detectorsignal of any other ionization chamber or group of ionization chambers;and switchable means for connecting the output signals from at least twogroups of ionization chambers to the adding means, said switchable meansconnecting the output signals of only one group of ionization chambersat a time.
 3. An array of detector elements as claimed in claim 2,further including means for time-integrating each output signal, andwherein the switchable means connects the integrated output signals tothe adding means.
 4. A computerized tomography method comprising thesteps of providing a plurality of detector devices arranged to measureradiation at different locations distributed around a patient position,projecting a substantially planar, fan-shaped distribution ofx-radiation through a sectional slice of the body of a patient, disposedat said patient position, from many different positions distributedangularly around said patient position, causing said detector devices toprovide electrical output signals indicative of the radiation emergingfrom said slice of the body of a patient along a plurality of mutuallydivergent directions from each of said positions, providing a processor,and selectively passing, to said processor, individual electrical outputsignals provided by individual detector devices or combinationelectrical signals generated by combining the electrical output signalsprovided by respective groups of neighboring detector devices, thenumbers of devices in such groups being selectable, and causing saidprocessor to process the signals selectively passed to the processor togenerate, in part from said electrical output signals or combinationelectrical signals selectively passed thereto, electrical data signalsindicative of the attenuation suffered by the x-radiation passingthrough the slice along various directions and to generate, in part fromsaid electrical data signals, a representation of the variation of anx-ray response characteristic over said slice.
 5. A device fordetermining local radiation absorption values in a slice of a bodyhaving a central axis, said device comprising:at least one radiationsource for generating a fan-shaped beam of radiation which irradiatesthe slice of the body and which passes through the slice in differentdirections, said radiation source being situated on a first side of thecentral axis; an array of detector elements, having uniform dimensionsand behavior, for detecting the radiation emitted by the radiationsource, said array being situated on a second side of the central axisopposite said radiation source, each of said detector elementsgenerating an output signal; means for forming group output signals aseither individual detector element output signals or as combinationoutput signals formed by combining the output signals of two or moreneighboring detector elements; and means for selecting the number ofdetector elements whose output signals form group output signals.
 6. Adevice for determining local radiation absorption values in a slice of abody having a central axis, comprising:at least one radiation source forgenerating a fan-shaped beam of radiation which irradiates the slice ofthe body and which passes through the slice in different directions,said radiation source being situated on a first side of the centralaxis; an array of detector elements, having uniform dimensions andbehaviour, for detecting the radiation emitted by the radiation source,said array being situated on a second side of the central axis oppositesaid radiation source, each of said detector elements generating anoutput signal; means for adding the output signals from a group of oneor more detector elements, the sum being a detector signal which iselectrically isolated from the detector signal of any other detectorelement or group of detector elements; means connected to the detectorsignal for time-integrating the detector signal; and switchable meansfor connecting the output signals from at least two groups of one ormore detector elements to the adding means, said switchable meansconnecting the output signals of only one group of detector elements ata time.
 7. A device as claimed in claim 6, wherein the detector elementsare ionization chambers.
 8. A device as claimed in claim 7, wherein thegroups of detector elements which are on and near a connecting linebetween the radiation source, central axis and array of detectorelements are smaller than the groups of detector elements which aresituated farther from the connecting line.
 9. A device as claimed inclaim 1, wherein the detector elements are ionization chambers.
 10. Adevice as claimed in claim 9, wherein the groups of detector elementswhich are on and near a connecting line between the radiation source,central axis and array of detector elements are smaller than the groupsof detector elements which are situated farther from the connectingline.